Movement happens across, not along
Wood moves almost entirely across the width of the board, barely at all along its length. A 300 mm wide oak panel can change by 2 mm or more between a humid summer and a dry winter; the same panel measured along its length will change almost imperceptibly.
This single fact drives almost every good design decision in solid wood furniture.
The growth rings tell you which way
Look at the end grain. A board with rings running vertically (quartersawn) moves about half as much as one with rings running parallel to the face (flatsawn).
Quartersawn stock costs more because it yields less from the log, but it is dramatically more stable — worth it for table tops and wide panels.
Never trap a panel
A solid wood top fixed rigidly to a frame at both ends will crack as it dries. The fix is to fasten it in a way that allows movement: slotted screw holes, figure-eight fasteners, or a breadboard end.
The same principle applies to drawer bottoms and cabinet backs, which should float in grooves rather than be glued around their full perimeter.
Grain direction decides the cut
Grain runs like a bundle of straws. Plane or cut in the direction the straws lie and you get a clean shaving; cut against them and the blade lifts the fibres and tears the surface.
When one board has grain running the other way, you have a choice: reverse the planing direction for that section, take a finer cut, or reduce the cut angle. All three are better than sanding out tearout, which never fully works.
